Protease inhibitors inhibit carcinogenesis in mouse skin. When Leupeptin and Antipain are injected during initiation by 30mg Beta Propiolactone, the number of tumors appearing after promotion with TPA is significantly diminished. When protease inhibitors Leupeptin, Tosyl Lysine Chloromethyl Ketone or Tosyl Arginine Methyl Ester are administered during promotion, the onset of tumors is delayed. Thus protease inhibitors modify development of tumors at both stages of two-stage carcinogenesis. The consumption of soybeans containing protease inhibitors as major food in the Japanese and Seven Year Adventists population may be responsible for the low breast cancer rate in these populations. We will attempt to confirm this hypothesis by feeding soybean trypsin inhibitor to mice in two-stage carcinogenesis experiments and to Sprague-Dawley rats exposed to ionizing radiation. The natural protease inhibitor occurring in seeds fulfills the requirement of a non-toxic protease inhibitor which is applicable to man.